The invention relates to a magnetic sensor for detecting metallic objects in a subgrade medium, i.e., for localization and/or depth of cover measurement of, for example, electrically conductive objects embedded in walls, floors and ceilings such as concrete reinforcing bars, copper pipe, prestressing cable, double T-beams, plates, grates, etc., in common subgrades of building structures, for example, concrete, brick, wood, plaster, etc.
A plurality of proposals is known within international patent class G01V3/08-12 for detecting objects buried in subgrades, wherein reference is made only as an example to European Patent Document No. EP 1092989 B1, German Patent Document Nos. DE 699 35 792 T2, DE 3615652 C2, DE 101 22 741 A1 and U.S. Pat. No. 5,557,206.
To detect objects in the sense outlined above, in principle a strong magnetic primary field is induced with a coil in the subgrade, which interacts with any metallic objects that are possibly located there. FIG. 1 of the drawings illustrates a general case, in which a primary or excitation coil 1 generates a strong periodic magnetic field 3, which, in the case of an object-free subgrade, does not generate a field asymmetry (Case A) in one or more of the receiving coils 2 that are situated symmetrically to the axis of the magnetic field 3 (Case A), while, in the case of the presence of a (ferrous) object 4, a registerable field change is detectable via the receiving coil 2 (Case B). This interaction is expressed, on the one hand, in that, in the case of permeable objects 4, particularly those made of iron, the magnetic flux lines are diverted in the direction of the iron due to flux guidance effects, because they follow the path of the least magnetic resistance. The field asymmetry that arises because of this can be measured differentially. If, for example, as depicted in Case B of FIG. 1, an iron object 4 is located in the subgrade, then the angle of the flux lines changes there and therefore the induced voltage measured by the right receiving coil. Therefore, an induced voltage difference unequal to zero originates between the left and right of the two receiving coils 2. This difference is a measure of the position and depth of the object 4.
On the other hand, a strong, rapidly decaying primary field 3 (see FIG. 2) generates eddy currents 5 in metallic objects 4, which generate a delayed, relatively weak and slowly decaying secondary field 6. The decaying eddy current magnetic field 6 can be measured by coils, in particular also via the excitation coil 1 that simultaneously serves as a receiving coil, and contains information about the position and depth of the object 4. In the case of pulse-shaped excitation, the eddy current response can be detected directly because the primary field 3 decays very rapidly (in a few microseconds), while the secondary eddy current field 6 decays considerably slower as a function of the depth of the object 4, the object size and the material properties of the object 4. Measuring the eddy current response is thus carried out after the primary field 3 decays. In the case of harmonic excitation, the secondary eddy current magnetic field 6 influences the primary field 3 so that field asymmetries again arise that causes the induced voltage changes as described above.
The induced voltage change may be measured both monostatically as well as bistatically. In the case of a monostatic measurement, the same coil is used to both excite as well as measure. In the case of a bistatic measurement, the excitation coil(s) and the receiving coils(s) are different (see only as an example DE 69935792T2, EP 1092989 B1, and DE 101 22 741 A1). The detection of objects embedded in building structure subgrades is carried out in the case of the hand-operated sensors described here by scanning or sweeping the suspected location of the embedded object multiple times. In this connection, scanning yields considerably more reliable localization results than with selective placement of the sensor.
Measuring instruments that are currently known for detecting metallic objects in a subgrade have comparatively simple coil arrangements having only a few coils. The possibilities for magnetomotive force in a subgrade are limited in this case with respect to the direction and shape of the magnetic field and therefore also the possibilities for detecting objects as well as for determining depth and position. Particularly measurement above welded grates, as it is used above all in concrete subgrades, causes problems because, in the case of a magnetomotive force from above (see FIG. 1), eddy currents are excited in at least an entire mesh, which can dominate as compared with the eddy currents generated within the individual lattice bars themselves such that localizing and securely detecting them is unreliable or impossible. In addition, permeable subgrades, such as, for example, brick containing iron oxide or concrete containing magnetite, also influence the magnetic flux guidance and therefore also the measurable induced voltage. Reliable localization and depth of cover measurement are generally impossible in such cases.
The object of the invention is therefore making available a coil arrangement and an operating method for detecting metallic objects in subgrades, which allow a localizing and/or depth of cover measurement of objects buried in the subgrade to be achieved considerably more precisely and reliably than was previously possible.
The invention started from the knowledge gained initially on the basis of experience with known measuring instruments that a good separation property with simultaneously high measuring depth is not possible with only one type of magnetomotive force, because coils with great measuring depth have poor separation properties. A separation of objects that are lying densely together at an average depth of between approx. 50 mm to approx. 80 mm is typically not realizable as a result. Narrow oval coil shapes such as those mentioned in DE 69935792 T2, particularly those with a high ellipticity, would indeed meet both requirements, but not the simultaneous requirement for rotational or orientational independency of the sensor, i.e., independence of the measuring results from the orientation of the coil arrangement related to the orientation of the embedded objects or related to the movement direction of the sensor. Even the reliability of the object localization and the depth of cover measurement with an improper orientation of the sensor are strongly restricted.
The invention provides a coil arrangement for magnetic sensors for detecting metallic objects in subgrades as well as discloses a measuring method that eliminates the aforementioned restrictions, such as they have been observed in the previously known measuring instruments of the type under discussion here. In particular, optimally adapted types of magnetomotive force, but also reception characteristics, are made possible for different measuring tasks, e.g., those that are sequential.
According to one embodiment, a magnetic sensor for detecting metallic objects in a subgrade medium includes a coil arrangement with a main coil, whose winding plane or winding surface or planes is (are) aligned essentially parallel to a movement surface of the sensor, wherein the subgrade medium experiences magnetomotive force perpendicular to the movement surface of the sensor. In this case, the movement surface is a sensor surface that, with proper use, is oriented essentially parallel to the subgrade medium and that is normally flat.
The plane on which the coil windings are projected perpendicularly are designated as the winding plane in the following. The winding surface designates the area in the winding plane surrounded by the projected windings. In the case of a cylindrical coil, the winding surface is its base area, i.e., the area which is bordered by the lowermost winding layer. The two-sided vertical projection of the winding surface defines an expanded winding cylinder, whose length is not limited to the length of the coil body and whose upper and lower base areas are defined parallel to the winding surface. The length of the winding cylinder may exceed the coil body toward the top and/or toward the bottom, for instance, symmetrically to a plane of symmetry of the coil body that is parallel to the winding plane. According to a group of embodiments, the winding cylinder can terminate with the upper edge and/or the lower edge of the coil body or of the winding body.
At least two coils of a first coil group are provided in the interior of the expanded winding cylinder of the first main coil, whose winding planes can be inclined at an angle of between −90° to +90° to the winding plane of the first main coil. According to one embodiment, the winding planes of the coils of the first coil group are aligned parallel to the winding plane of the first main coil. According to a further group of embodiments, the coils of the first coil group are located completely or predominately in the interior of the coil body of the first main coil. Independent of the actual angle between the winding planes of the first main coil and the coils of the first coil group, the coils of the first coil group are also designated in short as “parallel coils” in the following. The first coil group may include several coils, for example, an even number, such as two, four, six or more coils, which are respectively arranged in pairs opposite from one another on a main axis of the expanded winding cylinder of the first main coil. It is preferred that at least the coils of each individual coil pair be spaced apart equally from the main axis of the expanded winding cylinder of the first main coil. According to another embodiment, all coils of the first coil group have the same distance from the main axis.
Moreover, the coil arrangement includes at least two additional coils of a second coil group arranged in the interior of the expanded winding cylinder of the first main coil, whose winding planes occupy other angles with respect to the winding plane of the first main coil than the winding planes of the coils of the first coil group. According to one embodiment, the winding planes of the coils of the second and those of the first coil group are situated orthogonally to one another. According to another embodiment, the winding planes of the second coil group are situated orthogonally to the winding plane of the first main coil. According to a further group of embodiments, the coils of the second coil group are located completely or predominantly in the interior of the coil body of the first main coil. Independent of the actual angle between the winding planes of the coils of the first and second coil groups or between the winding planes of the coils of the second coil group and the main coil, the coils of the second coil group are also designated in short as “orthogonal coils” in the following. Similar to the first coil group, the second coil group may also include an even number of coils, for example two, four, six or more coils, which are respectively arranged in pairs opposite from one another on a main axis of the main coil. It is preferred that the coils of each individual coil pair be spaced apart equally from the main axis of the main coil. According to a further exemplary embodiment, all coils of the second coil group have the same distance from the main axis.
The coils of the first and second coil group may be smaller than the first main coil insofar as their winding surfaces may be smaller than the winding surface of the first main coil.
According to one embodiment, the main coil is designed as a rotationally symmetrical coil, for example as a rotationally symmetrical flat coil, wherein the main axis coincides with the axis of symmetry of the rotationally symmetrical coil.
To improve the sensor properties with respect to object localization, depth of cover measurement, detection of ferritic subgrades as well as grate structures, e.g., of welded grates, and determination of the orientation of embedded objects, it is advantageous, if respectively two of the at least two coils of the first coil group (parallel coils) are arranged opposite from one another and connected electrically with one another in such a way that their excitation currents run in the opposite direction of each other so that their magnetic fields flow through an object embedded in the subgrade between the two coils essentially horizontally and in the same direction. For example, the first coil group includes four coils (parallel coils) with the same winding plane at an angular distance of respectively 90° and equally spaced apart from the main axis of the main coil. To widen the horizontal magnetomotive force emanating from the smaller parallel coils, it can be advantageous to wind the coils of the first coil group ovally or elliptically, wherein the auxiliary axes of the coils are respectively aligned the same, opposing in pairs. The coils of the first coil group have, for example, respectively essentially the same winding surface and/or the same inductivity.
The second coil group includes for example four coils (orthogonal coils) of the same size and induction, whose winding surfaces are respectively aligned perpendicular to one another and parallel to the main axis of the sensor and thus parallel to the main axis of the main coil.
The coils of the second coil group may be arranged between the main axis and the coils of the first coil group. For example, four orthogonal coils may be arranged spaced apart close to one another inside the parallel coils, at the same distance and symmetrically to the axis of the main coil.
A particular advantage in terms of better detection, particularly of deeper lying objects, is produced if the coil arrangement includes a second main coil with a winding plane parallel to the first main coil, which is arranged relative to the movement surface of the sensor below, but preferably above the first main coil. The second main coil is, for example, a rotationally symmetrical coil, for instance a rotationally symmetrical flat coil, whose diameter is somewhat smaller or larger than the diameter of the (first) main coil so that distinctly different signal signatures are produced, which enable a better depth of cover estimation. For example, the diameter of the first main coil lies between approx. 50 mm and approx. 150 mm, preferably between approx. 80 mm and approx. 100 mm. The diameter of the second main coil is for example approx. 10 mm smaller or larger than the diameter of the first main coil.
For sensors of the type under discussion here that are larger in terms of surface, it can be advantageous for a further increase in resolution in the detection of objects in subgrades to provide an in-line arrangement of several of the coil arrangements described above. In particular, the coil arrangement includes an in-line arrangement of several, respectively equally grouped parallel coils and/or orthogonal coils, wherein this in-line arrangement is surrounded by one or two common main coil(s).
Further details of a coil arrangement and for operating a coil arrangement with the characteristics according to the invention are explained in the following on the basis of exemplary embodiments.